Identified via two-million-year-old fossils, a new human ancestor dubbed Australopithecus sediba may be the "key transitional species" between the apelike australopithecines—and the first Homo, or human, species, according to a new study.

"We've never seen this combination of traits in any one [early human species]," study leader Lee Berger told the journal Science, where the new study is published today.

Found in the remnants of an underground cave network in South Africa, the partial Australopithecus sediba skeletons are believed to be from a roughly 30-year-old woman and an 8- to 13-year-old boy.

The pre-human pair, who may or may not have been related, apparently fell to their deaths into a chasm littered with corpses of saber-toothed cats and other predators.

The new species may be the wellspring—"sediba" in the local Sotho tribal language—from which our ancestors flowed, the report suggests.

Berger, of the University of the Witwatersrand in Johannesburg, conjures a different metaphor.

"It's the opinion of my colleagues and I that [Australopithecus sediba] may very well be the Rosetta stone that unlocks our understanding of the genus Homo," Berger said in a statement, referring to the artifact that helped decipher ancient Egyptian hieroglyphics.

Growing to just over 4 feet (1.2 meters) tall, A. sediba has a number of key traits that some would say mark it as an early human, like Homo habilis, which many consider the first human species.

A. sediba, for example, had long legs and certain humanlike characteristics in its pelvis, which would have made it the first human ancestor to walk—perhaps even run—in an energy-efficient manner, the study says. (Related: "Did Early Humans Start Walking for Sex?")

Also, A. sediba's face had small teeth and a modern—rather than chimpanzee-like—nose, the study says.

And as in humans, the shapes of A. sediba's left and right brain halves—discernible from indentations on a remarkably preserved skull—appear to have been uneven.

A facial reconstruction is in the works, and many people will be surprised by how human the new fossil species looks, Berger predicted in a press conference Wednesday. "What you'll see, I suspect, is something surprisingly more modern than we would expect in ... other things that have been called Australopithecus," which translates to "southern ape."

So if our newest evolutionary ancestor is so human-like, why doesn't the new study classify it as human?

Berger's team believes that certain apelike traits force the new species into the Australopithecus genus, or group of species.

For one thing, unlike human species but like other australopithecines, A. sediba had a very small brain. The fossil species also had long ape-like arms with primitive wrists that were well suited for climbing trees.

Australopithecus Sediba's World

In what's now South Africa, A. sediba lived in a patchwork of grasslands and woods, where the fossil species likely ate fleshy fruits, young leaves, and perhaps small animals.

The generally flat landscape was broken up by small hills and cliffs, some of which contained caves, which could apparently be treacherous.

Scientists speculate that a harsh drought may have driven two desperately thirsty members of A. sediba to enter one of these caves in an attempt to find an underground source of water.

The pair may have clambered partway down into the cave, only to slip and fall several yards to their deaths. The deathtrap also contained fossils of 25 species that lived alongside A. sediba, including potential predators such as saber-toothed cats, hyenas, and wild dogs.

A. Sediba Only Human After All?

Other anthropologists seem to be unanimously excited about the new human-ancestor fossils. But not everyone is so sure the new species is the "key transitional species" between prehistoric apes and humans suggested by the study.

"I don't think there's a lot of compelling evidence to suggest that [A. sediba] lies between Australopithecus and Homo," said anthropologist Bernard Wood of George Washington University.

A. sediba "doesn't fit what our preconceptions would be about the ancestor of Homo," said Wood, who wasn't involved in the study.

For example, A. sediba's arms are too long—too apelike—and the species isn't as well adapted for upright walking as some scientists expect the direct ancestor to the first humans to be, Wood said.

Also, at 1.95 to 1.78 million years old, the A. sediba fossils simply aren't old enough to represent an ancestor to Homo, said anthropologist Brian Richmond, also of George Washington University. (Explore a prehistoric time line.)

"It's hard to argue this is the ancestor of Homo when it's occurring much later than the earliest members of the genus Homo by half a million years," Richmond said, referring to an early fossil of H. habilis that dates back to 2.3 million years ago.

Anthropologist William Kimbel thinks this chronological conundrum could be resolved by calling the new specimens Homo instead of Australopithecus.

"By putting it in Australopithecus and saying it's ancestral to Homo, you're left with having to wonder how to accommodate earlier Homo [species]," Kimbel said.

"If you put it in Homo, that problem falls away," he said. "It's then just one of several species around two million years ago that are near the base of the Homo lineage."

Susan Anton, an anthropologist at New York University and a joint editor of the Journal of Human Evolution, agreed.

A. sediba has so many similarities with Homo that "I think they might have been better off including it in Homo," Anton said.

"If you do that, then this is really no longer a transitional species between Australopithecus and Homo. It is Homo"—and just an evolutionary dead end in human ancestry.

With Fossils, Timing Is Everything

Berger, who has been funded in the past by the National Geographic Society, maintains that A. sediba belongs with other australopithecines because its anatomy suggests it was still climbing trees. (The National Geographic Society owns National Geographic News.)

"It hasn't made that grade-level shift to the genus Homo" yet, he said.

As for questions about its timing, Berger believes future discoveries could turn up A. sediba fossils that are hundreds of thousands of years older, which would make them old enough to be the ancestors of early Homo species.

"This [discovery] site is only a point in time. It doesn't represent the first appearance of this species, nor will it probably represent the last," he said.

Regardless of where A. sediba ends up in the human family tree, it's already an important fossil precisely because of all the questions that it raises, said paleontologist Scott Simpson of Case Western Reserve University in Cleveland, Ohio.

"This fossil is not one that resoundingly answers any specific questions," Simpson said. "What it does is reinforce the idea that we haven't even asked all the appropriate questions yet.